I found this here...
http://www.todayshomeowner.com/how-to-keep-tomatoes-from-splitting-open/
Splitting usually happens after a hard rain, and it’s caused by the sudden change in moisture. A rapid increase in water levels will cause the insides of the fruit to swell faster than the skin can stretch, resulting in cracks.
This problem is worse in hot temperatures and during drought, as the plants respond by producing fruits with tougher skins that can’t handle sudden rapid growth.
While you can’t totally prevent it, splitting will be reduced if you take steps to keep moisture levels consistent within the plants.
Tips to Reduce Tomato Splitting:
- Watering: Keep your tomato plants evenly and regularly watered, so that they aren’t as shocked by a hard rain. Avoid the cycle of drying out and then overwatering.
- Mulching: Applying mulch around your tomato plants helps to even out moisture levels.
- Fertilizing: Fertilize carefully, or use naturally-balanced compost, to prevent bursts of water-sensitive growth.
- Picking: If you don’t use extra irrigation and therefore can’t control the amount of water, you can go out right after the rain and pick as many tomatoes as you can, and allow then to ripen off the vine.
- Choosing Variety: Each variety of tomato is different – check out the varieties you’re planting to see if they’re more prone to splitting than others.
- Enjoy: Finally, keep in mind that as long as the fruits are clean and insect-free, there’s nothing wrong with a split-open tomato!
